Members of the flow control industry, such as producers of pipeline components for the transmission of water, gas, oil, or other fluids have focused substantial attention on the problem of creating and maintaining connections between adjacent lengths of pipe, or pipes and fittings, or pipes and valves. In applications where the fluid, such as water for fire mains or water distribution in municipalities, is under high pressure, various means are used to prevent separation of the joints between piping components. Piping components are joined to prevent separation caused by thrust forces, earth movement, and external mechanical forces exerted on piping components. These components include, for example, pipes, couplings, fittings, valves, and fire hydrants. The majority of the solutions can be categorized into either “push-on joints,” “mechanical joints,” or “flanged joints.”
Iron pipe has traditionally been used to withstand the large pressures that are necessary for municipal water systems and other systems. Those pressures are needed to carry fluids over long distances, to carry large amounts of fluids, and to prevent contamination of the systems in the event of a hole or other breach of the system. There are two related problems in the history of using pipes of any sort, including iron pipe: (1) creating a secure seal to join the pipes and to withstand large pressures, and (2) bending or deflecting the joints of the pipes to meet the intended use of the pipes.
The first substantial use of cast iron pipe was in Europe in the 17th century. The piping systems of the 17th and 18th centuries primarily had flanged ends that bolted together with lead or rawhide gaskets for sealing. Flanged joints continue to be used for some applications today but with rubber gaskets. Flanged joint systems are costly to install and require considerable maintenance.
The first bell and spigot joint was developed by Thomas Simpson of the Chelsea Water Company in England in 1785. The joint was caulked with jute rope impregnated with pine resin or tallow and sealed in place with molten lead. The bell and spigot joint remained the predominant pipe joint until the advent of the push-on joint, for example the TYTON® Joint, in 1956.
There are numerous methods of securing piping components in series to make up a pipeline, roughly divisible into three main categories: (1) rigid, as with bolted flange connections; (2) flexible, as with numerous designs such as TYTON® push-on joints and gaskets, or TYTON® combined with self-restraining gaskets bearing toothed inserts, such as FIELD LOK 350® Gaskets providing both sealing and autonomous restraint; and (3) others with a limited amount of incidental flexibility, such as PVC Pipe with Rieber Gaskets where minor flexibility is possible due to the plasticity of the gasket and pipe materials and to joint tolerancing.
Push-on solutions are exemplified by U.S. Pat. No. 2,953,398, and account for the majority of straight-run pipe connections. In a typical configuration, a spigot of a pipe slides into a bell of another pipe past a tightly fitted gasket. A variation of the push-on joint is evidenced by U.S. Pat. No. 2,201,372, which employs a compression snap-ring fitted within a special lip of the bell, in order to exert pressure onto locking segments and thus drive them into the spigot, restraining the joint against thrust forces. U.S. Pat. No. 3,445,120, likewise employs a gasket with toothed locking segments encased therein that are generally disposed such that they and the gasket may roll between a locked and a free position. As the gasket bulb rotates under extraction forces, it is intended eventually to encounter a position in which the segments must compress the gasket to allow further rotation of the segment and engagement of the teeth with the mating pipe spigot, thereby terminating the rotation and compression of the gasket bulb and restraining the joint.
Other examples of restrained push-on joints include those disclosed in U.S. Pat. Nos. 5,295,697, 5,464,228, and 5,067,751. In those references, the connection is effected by either locking segments or wedges within the gasket that engage the spigot. The locking segments possess a groove that mates with an annular rib on the bell, such that the rib acts as a rocker, or cam, or, alternatively, as a wedge. During insertion of the spigot into the bell, the segments rotate on the rib, but are prevented from appreciable axial movement by the mating of the rib and groove. Upon experiencing counter-forces tending to effect removal of the spigot, the rib acts as a cam, both causing the segments to pivot on the rib as an axis, and exerting a radially inward pressure as the segment attempts to slide past the rib. These types of joints depend on compressive force on the rubber gasket to maintain the connection of the pipes.
While the push-on type joint has obtained wide acceptance for pipe joints, acceptance for fittings, valves, and hydrants is much lower. The contours of bell sockets of the push-on joint require a high degree of precision for a cast surface. In restrained joints, an additional locking joint is necessary, which also requires a high degree of precision to manufacture. It often takes a high degree of skill and alignment precision, as well as substantial force (i.e. in the range of 600 to 800 pounds of force for an eight inch size pipe), to assemble joints using the above described push-on type joints. The insertion force with present push-on designs increases proportionally with conduit diameter. Moreover, insertion forces increase substantially in low-temperature conditions.
A current trend in the industry is to manufacture pipe with walls much thinner than the current designs. Whether the pipe end is produced in a manufacturing plant or is the result of field cuts required to adjust the length of the pipe, such pipes cannot realistically be beveled or have rounded ends. Damage to the gaskets or displacement of the gaskets is a likely outcome when inserting a spigot end of a pipe not properly aligned or without a beveled or rounded end into the bell of another push-on joint pipeline component. A further consequence of the high assembly forces required is that installers favor mechanical joint connections for fittings, valves, and hydrant shoes because they require lower assembly forces.
Attempts to design low-insertion-resistance joints have been made in the past, but these designs were not completely satisfactory because normal conical inner surfaces do not allow for sufficient deflection of the bell and socket joint. For example, U.S. Pat. No. 3,815,940 and U.S. Patent Application Publication No. 2009/0060635 both show bells with conical inner surfaces. Cast iron pipe is rigid and does not permit deflection. Creating ductile iron pipe permits slightly greater deflection at the joint. However, that deflection carries an increased risk of loss of pressure. If the joint is a straight connection, no deflection is possible. If the joint is connected into a cone-shaped annulus, deflection of more than a few degrees would degrade the connection opposite the angle of deflection, ultimately leading to a greater potential for a loss of pressure. Small movements in the earth could, over time, cause leaks in underground piping systems.
Thus, there is a need for a connection that is less sensitive to misalignment and temperature extremes, has reduced frictional resistance to the insertion of the spigot until the desired connection is achieved and the coupling is maintained, and maintains a seal under high pressures even if the joint is deflected.